Vertical Cavity Surface-Emitting Lasers (VCSELs) are currently being considered for use in CD-ROM drives, DVD heads, and laser printers. For all of these applications, the optics which deliver the laser power to the disc or drum have a certain degree of polarization sensitivity, such that if the polarization of the light source drifts, the drift will translate into detrimental output power variations. In addition, the optics can be complex enough in these systems that knowledge of the polarization direction is necessary to optimize the optics design. For these reasons, the light sources for such applications must have a stable fixed polarization, and the polarization direction must be the same for every laser fabricated.
VCSELs often have a fixed polarization while operating in the fundamental mode. However, the output light is, in general, a mixture of polarization states with ratio of the light in the fixed polarization state to the other polarization state typically of the order of 10:1. In addition, the polarization state is not always stable. For example, the polarization direction can often flip by 90.degree. as current is increased beyond threshold and as the laser switches between modes.
In addition to these instabilities, the polarization tends to be randomly oriented from device to device. This is due to the circular symmetry of typical VCSEL designs. Various methods have been applied to lock the polarization direction by adding some asymmetry to the cavity such as elliptical current confinement, or elliptical optical confinement, or both. However, these solutions have met with limited success in that the percentage of finished devices that remain locked in the desired polarization direction is significantly less than 100%. In addition, the beam shape is altered by some of the methods from the desirable circular output beam into a less desirable elliptical shape.
Broadly, it is the object of the present invention to provide an improved VCSEL.
It is a further object of the present invention to provide a VCSEL in which the output light is locked into one polarization state.
It is a still further object of the present invention to provide a VCSEL in which the polarization state is the same for all devices.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.